Looking at some sources the issue seems to be fairly complex, however it does indeed seem fairly easy relative to other countries like Canada to gain Japanese citizenship (though very few apply).
https://en.wikipedia.org/wiki/Immigration_to_Japan#Immigrant...
Looking at this and other citizenship guides, the main requirements seem to be that:
1. You live and work in Japan for 5 years (not counting student visas)
2. Are the age of majority in your country and over 20
3. Have not broken any laws while in Japan
4. Are able to support yourself financially and are willing to renounce other citizenships
It seems that about 90% of people that apply for naturalization are accepted. Yet less than ~10,000 are accepted every year, so applying for citizenship seems unpopular relative to other G7 countries.
Neat. I wonder why I was so certain that immigration was next to impossible and only visas were granted?